Measuring transducers of vibration-type comprise, usually, at least one oscillatable measuring tube for guiding the medium, wherein the measuring tube has bending oscillation modes, whose eigenfrequencies depend on the density of the medium. Based on a measured eigenfrequency, the density can then be determined. Compressible media oscillate relative to an oscillating measuring tube, which leads to a change of the eigenfrequency of the measuring tube compared with the eigenfrequency of a measuring tube that is filled with an incompressible medium of equal density. The closer the resonance frequency of the medium approximates the eigenfrequency of the considered bending oscillation mode, the greater is the change of the eigenfrequency. The consideration of the ratio of two eigenfrequencies offers, consequently, an approach for ascertaining the described influence of the compressibility of the medium and for correcting a density or flow measurement correspondingly.
The as-yet unpublished patent applications DE 10 2015 122 661.8 and DE 10 2015 112 737.7 disclose corresponding methods for ascertaining a physical parameter of a medium containing gas, or a medium which is a gas. In both methods, the eigenfrequencies of the f1 mode and the f3 mode are evaluated, to ascertain the velocity of sound in the medium, or the thereto proportional resonance frequency of its oscillations relative to the measuring tube of a measuring transducer, in order on this basis to be able to ascertain and correct the influence on the eigenfrequencies of the two modes. The described methods are applied especially in the case of measuring transducers with two parallelly extending measuring tubes, which oscillate oppositely to one another.
Known, for example, from DE 10 2011 010 178 A1 as measuring transducers for greater flow rates are measuring transducers having two curved measuring tube pairs. In the case of these measuring transducers, the measuring tubes are mechanically coupled, so that the two measuring tube pairs oscillate with essentially the same frequency. Thus, these measuring transducers behave like measuring transducers with only one measuring tube pair. The measuring transducer according to DE 10 2011 010 178 A1 has a compact design, wherein the two curved measuring tube pairs have equally sensed courses. The two measuring tube pairs have because of different mass distributions per se significantly differing eigenfrequencies. Nevertheless, to get to a shared eigenfrequency, substantial couplers are provided between each two tubes. This interferes with excitation of a stable f3 mode.